OBJECTIVE: To study and to compare the status of serum folic acid in three groups of infants and to evaluate the need of supplying this vitamin in 4-month old infants, when tissue reserves are low and dietetic resources (exclusive milk feeding) are poor in folates. METHODS: Serum folic acid levels were analyzed by radioimmunoassay method in 120 eutrophic infants with four months of age. These infants were exclusively milk fed. Among them, 40 were nourished with human milk, 40 with boiled pasteurized cow milk and 40 with industrialized milk formula enriched with folic acid. RESULTS: Folic acid levels in infants exclusively nourished with boiled and pasteurized cow milk were significantly lower than those found in infants fed with either breast milk or formula enriched with folates. These last two groups did not show differences between them. CONCLUSIONS: The study shows the need of the folic acid supplementation in 4-month old infants exclusively nourished with boiled pasteurized, since its use can led to deficiency of this vitamin so important in human nutrition.